In advanced process technology, Negative Bias Temperature Instability (NBTI) effect is a significant degradation source of transistors' threshold voltage (Vt) and currents (Idsat/Idlin). NBTI manifests as an increase in the threshold voltage (Vt) and consequent decrease in drive currents (Idsat/Idlin). Due to the NBTI degradation on transistors' Vt and Idsat/Idlin, the affected transistor's speed is reduced and may cause significant timing issues, such as max delay paths and detrimental min-delay paths (i.e., delay mismatch between generating and sampling paths).
In particular, NBTI can happen when a PMOS transistor undergoes a constant stress, such as in a clock gating (e.g., not allowing a clock signal to pass through) or standby mode (i.e., not in active operation) situation in an effort to reduce chips' dynamic power consumption. For example, in some applications, a relatively large inverter driver and an output node are parked at a logical 1 state using an operational PMOS transistor during a clock gating or standby mode situation.
Conventional methods to deal with NBTI includes: 1) guard banding (i.e., taking out some initially available operation frequency, e.g., shipping a chip at 1 GHz when 1.2 GHz is initially available), 2) gate sizing, and 3) Vdd and Vt tuning, etc. However, the drawbacks for these methods include: 1) sacrificing chips' initially available performance by as much as 10-15% for guard banding, 2) an area overhead of 10-20% and the accompanying power consumption increase for gate sizing, and 3) aggravating NBTI degradation for Vdd/Vt tuning.
Accordingly, new circuits and methods are desired to solve the above problems.